Many thanks. That did the trick. The solution to remove the borders around the Site brand, while leaving those of the top menu and the main navigation menu is as follows:

/* removes borders around site brand */
.site-branding {
border: hidden;
}
/* removes backround which appears as second set of borders round the site brand */
.site-branding:before, .site-branding:after{
background-color: transparent;
}

Thanks for the reply and the adivce.

I have managed to disable the header image on specific pages/posts by using the CSS rule you gave me with the addition of the page/post ID. For example:

.page-id-36 div#page-site-header {
display :none;
}

But I have not been able to come up with a rule that will resize the featured image.

The site where I am using Magpaper is the following: https://historyandtranslation.net

I would like to resize the featured image to half the height, while keeping the width as it is.

Thanks, Chris